A contact-angle measurement method for measuring the contact angle of a given droplet on a sample surface is known (see Patent Document 1, for example). The contact-angle measurement method disclosed in Patent Document 1 is a method for obtaining a contact angle (φ) from a sample surface (3), a gaseous environment, and a droplet (5) that is located on the sample surface (3) and that includes a curved surface (7), the axis of symmetry (ASy-ASy), and a predetermined volume (VI). This contact-angle measurement method includes steps (a) to (d). In step (a), an image of a subject is formed based on the reflection properties of the surface (7) of the droplet (5), the position of the subject relative to the optical axis (A-A) of the optical measurement system and the position of the subject relative to the sample surface (3), which are both known, and the axis of symmetry (ASy-ASy) of the droplet (5) that is located on or in the vicinity of the optical axis (A-A). In step (b), the distance between the image of the subject and the axis of symmetry (ASy-ASy) of the droplet (5) is obtained. In step (c), the radius of the curved surface of the droplet (5) is obtained by using the measured distance. In step (d), the contact angle (φ) is obtained by using the radius of the curved surface.